The present invention relates to apparatus and methods for exchanging heat between various fluids and, more particularly, apparatus and methods for exchanging heat between fuel, fan-air, and bleed-air in an integrated heat exchanger.
With development of de-oxygenated jet fuels, which is burned in engines, the fuel can be used for additional heat sink capacity by taking the fuel above normal coking limits of about 350° F. Using additional heat sink capacity of the fuel in lieu of fan-air can reduce required cooling air flow and reduce aircraft drag. Additionally, delivering higher temperature fuel to the combustors can improve engine specific fuel consumption (SFC).
One system within aircraft, which needs cooling and operates at higher temperatures, is an engine bleed air system. Typically ram air or engine fan air is used to cool the bleed air from around 1000° F. to under 400° F. By using engine fuel flow to cool the bleed air, the amount of ram or fan air used in cruise can be reduced with engine and aircraft performance benefits. A bleed air pre-cooler is normally sized for a ground or hold case where fuel flow is low such that there is still a need for air cooling of the bleed air.
Stainless steel or Inconel plate fin heat exchangers are typically used for bleed air pre-coolers because of the need for very compact designs. To reduce the risk of fuel leakage into the hot bleed air, double header bars plus a buffer passage have been used in other liquid to air plate fin designs. This imposes a large weight and/or size penalty due to the poor thermal conductance of the buffer passage.